The present invention relates to methods of and apparatus for liquefying a gas such as natural gas, utilising a refrigerating mixture cycle fluid with at least one principal cycle fluid under a pressure referred to as low cold-producing pressure, in counterflow heat exchange with a heat-producing fluid which is to be liquefied on the one hand, on the other hand with at least one heat-producing principal cycle fluid under a pressure referred to as high pressure, with addition to a said principal cycle fluid under a low pressure and at an intermediate heating temperature of at least one secondary cycle fluid obtained by withdrawal of a part in the liquid state of the said refrigerating mixture at a higher pressure than its low pressure, which is expanded to a low pressure before the said addition.
In known embodiments of this method, use has always been made of exchangers of the coiled type positioned in line, allowing for withdrawals and additions of fluid at the cold end of each exchanger. The redistribution of fluids under low pressure however requires comparatively complex arrangements which are very costly in any event. By contrast, the exchangers of the type comprising stacks of plates and tie bars forming:
at least one plurality of uninterrupted longitudinal compartments forming passages for such a principal cycle fluid under low pressure, the said compartments extending from a cold end of the said exchanger; PA0 at least one plurality of longitudinal compartments forming passages for at least one of the following fluids, being at least one such principal cycle fluid under high pressure and the said fluid which is to be liquefied, the said compartments terminating at a cold end of the said exchanger, PA0 at least one plurality of uninterrupted longitudinal compartements forming passages for such a principal cycle fluid under a low pressure, said compartments extending from a cold end of said exchanger; PA0 at least one plurality of longitudinal compartments forming passages for at least one of the following fluids, being at least one such principal cycle fluid under a high pressure and the said fluid which is to be liquefied, the said compartments terminating at a cold end of said exchanger: PA0 the said addition of a secondary fluid to a principal fluid occurring in distributed manner in each low-pressure compartment in at least one transverse volume extending along the thickness of the exchanger and situated at heat exchange distance from the inlets and outlets of the said compartments forming passages for a principal fluid under a low pressure.
are of comparatively inexpensive construction, but in this case also it is incumbent on the user to provide several exchangers in line, for the re-introduction of the secondary fluids. It is for this reason that, if it has been decided to utilise a single exchanger only providing for liquefaction of the gas processed by itself, for the purpose of cost limitation, it has been sought to provide a simplified method lacking the forming of secondary cycle fluid requiring re-introducing actions, but the application of a method of this kind is comparatively difficult since its specific energy is high.
Accordingly, it is an object of the invention to provide a method of gas liquefaction which simultaneously offers the advantages of a single line exchanger for the complete temperature gradient up to liquefaction of the gas which is to be processed and of a method utilising at least one withdrawal and re-introduction of a secondary cycle fluid.